Abstract: A special vapour cell has been built, that allows the measurement of theatom-surface van der Waals interaction exerted by a CaF2 window at theinterface with Cs vapour. Mechanical and thermal fragility of fluoride windowsmake common designs of vapour cells unpractical, so that we have developed anall-sapphire sealed cell with an internal CaF2 window. Although impurities wereaccidentally introduced when filling-up the prototype cell, leading to aline-broadening and shift, the selective reflection spectrum on the Cs D1 line894 nm makes apparent the weak van der Waals surface interaction. Theuncertainties introduced by the effects of these impurities in the van derWaals measurement are nearly eliminated when comparing the selective reflectionsignal at the CaF2 interface of interest, and at a sapphire window of the samecell. The ratio of the interaction respectively exerted by a sapphire interfaceand a CaF2 interface is found to be 0.55 $\pm$ 0.25, in good agreement with thetheoretical evaluation of ~0.67.